1. Field of the Invention
This invention relates to a slip control system for a vehicle such as a traction control system or an antiskid control system. This invention also relates to a rough road detecting system for determining whether the vehicle is running a rough road in order to change the manner of the slip control according to whether the vehicle is running a rough road or a smooth road.
2. Description of the Prior Art
There have been put into practice slip control systems for a vehicle such as a traction control system and an antiskid control system.
In the traction control, the slip ratio of the driving wheels of the vehicle is detected, and when the slip ratio exceeds a predetermined threshold value, the engine output power and/or the braking force are controlled (the engine output power is reduced and/or the braking force is increased) so that the slip ratio of the driving wheels converges on a target slip ratio in order to prevent the driving wheels from slipping due to excessively large driving force, for instance, during acceleration, thereby ensuring better acceleration performance.
In the antiskid control, the slip ratio of the wheels of the vehicle is detected, and when the slip ratio exceeds a predetermined threshold value, the braking force is controlled (reduced) so that the slip ratio of the driving wheels converges on a target slip ratio in order to prevent the wheels from locking.
Generally the slip ratio of each wheel is determined on the basis of the peripheral speed of the wheel. Accordingly, in the case of the traction control system, when the vehicle is running a rough road and the peripheral speed of each wheel fluctuates due to uneveness on the road surface, the slip ratio can frequently exceed the predetermined threshold value and the traction control can be performed when the running condition of the vehicle does not require the traction control, whereby the driving force is frequently reduced and the acceleration performance deteriorates.
In the case of the antiskid control system, a more efficient braking can tale place when locking tendency of the wheels is enhanced so that the wheels catch the uneveness on the road surface than when the antiskid control is performed and the slip ratio is converged on the target slip ratio so long as the vehicle is running a rough road.
Thus, in either of the traction control and the antiskid control, it is preferred that the threshold slip ratio be higher to a certain extent when the vehicle is running a rough road than when the vehicle is running a smooth road so that the traction control or the antiskid control becomes less apt to be performed.
In order to change or correct the threshold slip ratio when the vehicle is running a rough road, whether the vehicle is running a rough road must be determined. To correct the threshold slip ratio when the vehicle is running a rough road will be referred to as "the rough road correction", and the means for detecting that the vehicle is running a rough road will be referred to as "rough road detecting means" or "rough road detecting system", hereinbelow.
An example of the rough road detecting system is disclosed, for instance, in Japanese Unexamined Patent Publication No. 64(1989)-29636.
However, with respect to the driving wheels, it is not always preferable to effect the rough road correction when the vehicle is running a rough road. That is, with respect to the driving wheels, the rough road correction should not be effected depending on the running condition of the vehicle (e.g., state of steering, vehicle speed, amount of depression of the accelerator pedal, running mode and the like).
Further preferably the contents of the rough road correction, e.g., how much the threshold slip ratio is to be increased, is determined depending on the running condition of the vehicle.